ABC News(PHOENIX) -- Accused murderer Jodi Arias borrowed two five-gallon gas cans from a former boyfriend the day before she drove to Arizona to kill another ex, Travis Alexander, according to testimony in Arias' murder trial Tuesday.

In cross-examination, prosecutors also forced Arias' former live-in boyfriend Darryl Brewer to describe his sex life with Arias as "pretty aggressive."

Brewer, 52, dated Arias for four years and shared a home with her in California for two years. He told the court Tuesday that Arias called him in May 2008, asking to borrow gas cans, but would not explain why. She called him again at least two more times, and arrived at his house on June 2008 to borrow the cans.

On the day she picked up the gas cans she told Brewer that she was going to visit friends in California and Arizona.

Prosecutors argue that Arias then drove to Mesa, Ariz., where she allegedly had sex with Alexander, took nude photos of him, and then stabbed him 27 times, slashed his throat, and shot him twice in the head. She is charged with murder and could face the death penalty if convicted.

Brewer said that Arias never returned the gas cans. The pair had been broken up two years earlier and they had only spoken "sporadically," he said.

Prosecutors also showed receipts from Arias' trip from her California home to Alexander's home in Mesa, showing that she purchased a 10 gallons of gas at one gas station the night before she drove to Arizona, and then another 10 gallons from a different gas station 10 minutes later. Prosecutors are expected to argue she brought the gas with her to fill up her car secretly on the way to Alexander's home, showing premeditation for the murder.

Arias' attorneys called Brewer as one of their first witnesses as they began mounting their case that Arias killed Alexander in self-defense, arguing that Alexander was controlling and abusive toward Arias.

They asked Brewer to explain how he and Arias had been in a stable relationship for four years, from 2002 to 2006, and had bought a home together before Arias met Alexander at a business conference and began to change.

"I saw a lot of changes in Jodi. She became a different person than I had known previously," Brewer said, describing how Arias' behavior changed in May 2006 when she joined a company called Pre-Paid Legal. She met Alexander and began seeing him. She continued to live with Brewer.

"She had continued to pay the mortgage, but she was not paying other household bills, she began getting into debt or financial trouble," Brewer said. "For me it seemed she was not as rational or logical."

Arias also converted to Mormonism while living with Brewer, telling him that he could no longer curse and she would no longer have sex with him because she was saving herself for marriage.

The pair had previously had an "enthusiastic" and "aggressive" sex life, Brewer admitted to prosecutors. Arias even purchased breast implants in 2006, he testified.

Brewer said that after Arias began to change, he made arrangements to move closer to his son from his first marriage, and he and Arias broke up.

They kept in touch with occasional phone calls until Arias asked to borrow the gas cans in June 2008, and then called him a week after borrowing the cans to say that her friend had been killed.

Martinez, reading notes from an interview Brewer gave to authorities during the investigation into Alexander's death, asked if Arias had ever mentioned needing an "alibi." Brewer said he did not recall any conversation about alibis.

"After this date of June 4, 2008," Martinez asked, "you received a call from Jodi Arias, and she was very agitated?"

"She was sad," Brewer said.

"Did she tell you that her friend had been killed and she did not have an alibi?"

"I don't remember that," Brewer said.

Arias was arrested a month after Alexander was found dead, in July 2008.